Well, the idea behind it, opening to a chest in the nevernever, is solid, that should work nicely.
I think it should definitely be an aspect at the very least. There's some great opportunity for compels.
Mechanically, the question really is, what is supposed to be the actual benefit? Being able to carry around a lot is nice, sure, but that alone is not necessarily worth any power.
If you don't want to spend any refresh on it, I would take it just as an aspect and if you want to have something that's a bit uncommon to carry around, spend a fate point to declare that you have it.
I could, for example, see it being done as a power like this.
Bag of Holding [-1]: Once per scene, you may use your Resources skill instead of any other skill, by taking out an item of the bag of holding that you could use to solve the problem at hand.
Since resources is not only money, but equipment, this would fit a character who would make use of an item like this.
As an enchanted item, it could be done similar, though less powerful.
Bag of holding (4 shift resources skill replacement "I've got just the right tool for the job", 1 enchanted item slot, 1 use/session)
This would allow you to use the item to place the "I've got just the right tool for the job" aspect on yourself, that you can invoke on any number of rolls. Including tagging it for effect, to turn it into something more detailed than "the right tool". The item assumes you have at least a lore of 4.